From ff867666106fa85bd91341beb9e8121b2f99bc5f Mon Sep 17 00:00:00 2001
From: Gabriel Schneider <gbrls@0x4200.cafe>
Date: Mon, 28 Sep 2026 18:58:24 -0300
Subject: ^ in a pattern with \n matches at every line start, and a ^ or $ that
 cannot work is refused

A pattern naming a newline runs over the whole text, where mvzr's ^ is only the text's start: /^def .*\n/ from #0 failed, and Edit ,x/^def .*\n/.../ silently did nothing. A leading ^ is now tried at each line start from the search's own; $ just before \n is dropped, as it changes nothing; any other ^ or $ in such a pattern is refused with why, since mvzr would read it as the text's ends. ^ inside an alternation holds only where a search starts, an mvzr limit, documented.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 src/regexp.zig | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

(limited to 'src/regexp.zig')

diff --git a/src/regexp.zig b/src/regexp.zig
index fe68585d..9c34ae2c 100644
--- a/src/regexp.zig
+++ b/src/regexp.zig
@@ -41,7 +41,12 @@ pub const Regex = struct {
     /// patterns take 45-80 ms, and a Debug build is ten times slower.
     pub const budget: u64 = if (builtin.mode == .Debug) 4_000_000 else 32_000_000;
 
-    pub fn compile(pat: []const u8) error{Bad}!Regex {
+    pub const e_anchor = "in a pattern with \\n, ^ can only come first and $ only just before a \\n";
+
+    /// `Anchor`: a pattern that names a newline has `^` other than first,
+    /// or `$` other than just before a `\n`, which mvzr would read as the
+    /// ends of the whole text and so never match where sam would.
+    pub fn compile(pat: []const u8) error{ Bad, Anchor }!Regex {
         if (pat.len == 0) return error.Bad;
         var buf: [256]u8 = undefined;
         var len: usize = 0;
@@ -68,6 +73,12 @@ pub const Regex = struct {
                     in_class = true;
                 } else if (c == '.' and spans) {
                     piece = "[^\\n]";
+                } else if (spans and c == '^' and i != 0) {
+                    return error.Anchor;
+                } else if (spans and c == '$') {
+                    // `x$\n` is `x\n`; any other `$` would be the text's end.
+                    if (std.mem.startsWith(u8, pat[i + 1 ..], "\\n")) continue;
+                    return error.Anchor;
                 }
                 if (!emit) continue;
                 if (len + piece.len > buf.len) return error.Bad;
@@ -91,6 +102,20 @@ pub const Regex = struct {
         mvzr.steps_left = rx.steps;
         mvzr.exhausted = false;
         defer rx.steps = mvzr.steps_left;
+        // A `^` pattern that spans lines: mvzr's `^` is its haystack's start,
+        // so each line start from `from` on is tried as one.
+        // ponytail: a search per line start, each to `hi`; the step budget
+        // bounds it.
+        if (rx.spans and rx.bol) {
+            var s = if (from == 0 or text[from - 1] == '\n') from else (std.mem.indexOfScalarPos(u8, text[0..hi], from, '\n') orelse return null) + 1;
+            while (s <= last and s <= hi) {
+                const hit = rx.re.match(text[s..hi]);
+                if (mvzr.exhausted) return error.TooSlow;
+                if (hit) |m| if (m.start == 0) return .{ .start = s, .end = s + m.end };
+                s = (std.mem.indexOfScalarPos(u8, text[0..hi], s, '\n') orelse return null) + 1;
+            }
+            return null;
+        }
         var start: usize = if (rx.spans) 0 else if (std.mem.lastIndexOfScalar(u8, text[0..from], '\n')) |nl| nl + 1 else 0;
         var at = from - start;
         // `^` cannot match in the middle of a line.
@@ -136,6 +161,20 @@ test "lines are haystacks: ^ and $ at each line, . never a newline, \\n spans li
         try std.testing.expectEqual(c.end, m.end);
     }
     _ = try Regex.compile("a.*a\\nq");
+    // `^` at every line start and `$` before a newline, when a pattern
+    // spans lines; anywhere else they are refused, never silently wrong.
+    const defs = "x = 1\ndef a\n\ndef b\n";
+    var def = try Regex.compile("^def .*\\n");
+    const d = (try def.find(defs, 0, defs.len, defs.len)).?;
+    try std.testing.expectEqual(@as(usize, 6), d.start);
+    try std.testing.expectEqual(@as(usize, 12), d.end);
+    try std.testing.expectEqual(@as(usize, 13), (try def.find(defs, 7, defs.len, defs.len)).?.start);
+    var blank = try Regex.compile("^\\n");
+    try std.testing.expectEqual(@as(usize, 12), (try blank.find(defs, 0, defs.len, defs.len)).?.start);
+    var dollar = try Regex.compile("1$\\n");
+    try std.testing.expectEqual(@as(usize, 4), (try dollar.find(defs, 0, defs.len, defs.len)).?.start);
+    try std.testing.expectError(error.Anchor, Regex.compile("(^|\\n)def"));
+    try std.testing.expectError(error.Anchor, Regex.compile("a$\\nb$"));
     var none = try Regex.compile("zzz");
     try std.testing.expect(try none.find(text, 0, text.len, text.len) == null);
     try std.testing.expectError(error.Bad, Regex.compile(""));
-- 
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